The present invention relates to a field-effect transistor (FET) in which electric conductivity of a semiconductor is controlled by a voltage applied from outside and, more particularly, to a mobility-modulation field-effect transistor in which mobility of carriers in channels is modulated by the external voltage i.e., the relative density of carriers in the channels is changed by the external voltage.
In general, field-effect transistors (hereinafter called FETS) are widely used as one of the most important devices in both discrete and integrated circuits because of their excellence in switching and amplifying functions and simplicity in construction. The operating principle of this FET is to modulate the density of carriers travelling within passages or channels provided in the semiconductor to conduct electricity by the action of a signal voltage applied to gate electrodes to thereby control the electric conductivity between the source and the drain electrodes. Generally, the channel transit time of carriers, that is the time .tau..sub.t required for the carriers to travel between the source and the drain electrodes, is equal to the channel length L divided by the carrier velocity v, that is the value L/v. Since the carriers within the channels are supplied from the source electrodes and the drain electrodes on opposite ends of the channels, it is required for full accomplishment of modulation of the carriers that the minimum change time of the signal voltage applied to the gate electrodes is of a substantially equal order to the channel transit time of the carriers. In order to reduce the response time of the FET, accordingly, it is essentlal to increase the carrier velocity v while reducing the channel length L. However, even in a gallium arsenide Schottky field-effect transistor GaAsMESFET known as a high-speed device, the ultimate limit of the channel transit time .tau..sub.t of the carriers is of the order of 1 picosecond (10.sup.-12 second) because the maximum carrier velocity is of the order of 2.times.10.sup.7 cm/s and the minimum channel length is of the order of 0.2 .mu.m. That is, in FETs, as long as carrier density modulation is utilized, it is impossible in principle to make the response speed 1 picosecond or lower.
Accordingly, conventional FETs have the disadvantage that the carrier density modulation within the channel is not performed sufficiently for an electric signal changing rapidly in a time period shorter than the minimum time period required to increase or decrease the carrier density within the channel, whereby the conductivity becomes unchangeable.